(a) Field of Invention
This invention relates to dryer fabrics woven entirely of monofilament polymeric warp and weft strands and more particularly to joining the ends of such fabric with a woven pin seam to form an endless belt.
(b) Description of Prior Art
A dryer fabric is usually woven as a long, wide, flat single piece and when it is installed on a paper machine the ends are joined by means of a plurality of loops provided at each end edge which are interdigitated to form an elongated passageway through which a pintle pin is inserted to form a hinge-type joint known in the paper-making trade as a pin seam.
There are several known methods of providing the plurality of loops at the end edges of the fabric and the one which is generally accepted as being the most satisfactory for monofilament dryer fabrics is the woven pin seam.
To make a woven pin seam ia predetermined number of weft strands is removed at each end of the fabric to leave projecting crimped warp strands. Alternating projecting warp strands are folded back and interwoven, in an adjacent position, into a plurality of added weft strands which are crimped similarly to those that were removed, thus forming a reconstituted woven area at each end of the fabric. In the process, selected ones of the folded back warp strands are bent around a loop-forming rod placed near the last added weft strand to form pintle loops and the remaining folded back warp strands are bent around the last added weft strand to form retaining loops. All woven back warp strands meet the ends of adjacent warp strands which have been woven part way into the group of added weft strands and, when clipped off closely to the surface of the fabric, form abutting termination points at various distances from the last weft strand in a predetermined uniform pattern throughout the reconstituted fabric of the seam area.
The advantages of the woven pin seam are that the continuity of the mesh at each end of the fabric, right up to the pintle loops, is maintained without a discernable mesh blockage and without a layered thickness where the fabric is folded back and sewn to provide loops in a de-wefted section or a specially prepared short section of fabric, provided with loops, is sewn to each end of the dryer fabric. Also the strength of the woven pin seam is reasonably controllable, being determined by the number of the looped warp strands and by the number of added weft strands through which the crimped warp strands are interwoven.
It is characteristic of the conventional round plastic warp strands of a heat set monofilament synthetic fabric to be permanently crimped and when these strands are unwoven the crimp remains and resists attempts to alter its configuration. Therefore, the success in making a satisfactory woven pin seam in the conventional fabric depends to a large extent on the pre-set crimps of the folded back projecting warp strands being substantially compatible with the crimp pattern of the fabric. This in turn depends on the crimp pattern of the warp being symmetrical so that when the warp strands are folded back and interwoven and are therefore reversed and inverted, their crimps will still follow the crimp pattern of the cut-back warp strands of the fabric with which they are aligned. This concept is explained in U.S. Pat. No. 4,026,331 which is restricted to a woven pin seam in single layer forming fabric in which the folded back warp strands have an inherent crimp which is compatible with the crimp pattern of the fabric. In those cases where the top crimps are different in length from the bottom crimps, but are otherwise symmetrical, each projecting warp strand may be rotated 180.degree. about its longitudinal axis to render the crimp of the folded back strand compatible. Some adjustment is usually made at the loop of each strand and this will influence the length of the loop.
It has heretofore been very difficult, if not impossible, to make a woven pin seam in fabric having a complicated and unsymmetrical mesh pattern. This difficulty is acknowledged in U.S. Pat. No. 4,206,787 commencing at Column 1, line 61. In these fabrics the warp crimp configuration at the top surface of the fabric may be quite different from that at the bottom surface and the up-slope of the crimp may be different from the down-slope within the fabric so that when the warp projections are looped and, even inverted, they will not fit readily into the crimp pattern when re-woven into the reconstituted fabric of the seam area. In some cases it may be possible to strain the crimps so that the re-woven ends of wrap can be forced into place but this usually results in the formation of some pintle loops that are either too long or too short and it can also result in gross distortion of the cloth at the seam area. When alternately long and short pintle loops are symmetrically formed without gross distortion of the fabric, two pintles may be used to join the ends of the fabric, each one passing through the short loops of one end and the long loops of the other end of the fabric. The use of two pintle pins, however, is a disadvantage, particularly in the case of a wide fabric because of the difficulty of inserting both pintle pins. In cases where there are minor differences in pintle loop lengths the pin seam is weakened because of the tendency for the stress to be taken only by those loops which are shorter in length.
In making a pin seam according to the known method described above, it is usual practice for pairs of consecutive warps to form retainer loops and alternate pairs of consecutive warps to form pintle loops. In a symmetrical mesh pattern, such as for example, a 4-shed duplex weave pattern having two layers of weft, the weaving back of the looped warp strands is not too difficult and the loops formed from alternate pairs of warps are substantially the same length because similar pairs form similar loops. In the case of non-symmetrical 4-shed weaving patterns, difficulty in weaving back can usually be overcome by rotating the strands at the loops. In the case of some meshes, however, weaving back and maintaining even loop sizes is prevented by substantial differences in crimp configuration at the loop. This difficulty is particularly apparent, for example, in 6-shed double layer-fabric in which the weaving pattern is repeated every third pair of warp strands and when every second pair is taken to form either a retaining loop or a pintle loop it is found that similar pairs must form retaining loops and pintle loops alternately. Thus, in order for the crimps of the looped warp to be more or less compatible with the crimp pattern of the fabric, there will be some variation in the lengths of each type of loop making it virtually impossible to obtain a satisfactory seam and certainly not a single pintle seam.
It has unexpectedly been found that when the warp is flattened according to the aforementioned patent application, Ser. No. 140,475, it can be woven back into the added weft strands of the end portion of the fabric without regard to the crimp configuration, without having to be twisted at the loops and also without variation in loop size. The present invention therefore is to provide a single pintle woven pin seam in all-monofilament dryer fabric having flattened warp strands.